1. Field of the Invention
This invention pertains to vibrating tables used as equipment on the presses utilized in the manufacturing of concrete products such as, for example, paving material, blocks, curbs, and other items.
2. Description of the Prior Art
Conventional presses are generally equipped with a concrete vibration device which is itself conventional and consists of a single table driven in a vertical direction by vibrators which use eccentric weights, which in turn are synchronized, and are rotated by motors, particularly electrical motors.
In these devices, the force of the unidirectional vertical vibration cannot be adjusted other than by the rotational speed of the eccentric weights during the course of the various phases of the vibration cycle of the manufacturing process for a product. Thus, the level of the vibrational force, the range of movement of the table, and the acceleration applied to the mould and the concrete which it contains, are determined solely by the single rotational speed. This speed is possibly subject to modification, but is always permanently set, and cannot be adjusted in any other manner.
Since the machines are almost always multi-purpose, they are designed to produce a very wide range of products. Quite often it is impossible to achieve favorable production conditions for different products, both with respect to quality of the product and with respect to an optimal length of time for the production cycle.
Disadvantages of the current state of the prior art are that the adjustments made with respect to certain phases of production (pre-vibration time, number of pulses by the filling compartment agitator, or even the duration of final vibration) are insufficient and inadequate.
It was concluded that action must be taken with respect to the prior art vibration devices themselves, since they have too many limitations and inadequacies. These prior art devices should be modified and improved to comply with the requirements for the production of a certain level of quality, using optimal conditions which reduce the final cost.
It is often the case with conventional facilities that, with an average complete cycle of 10 to 15 seconds, the length of the pre-vibrational and vibrational phases is on the order of one-third of the length of the complete cycle. The process consists of starting the motors, possible stopping, and shutdown before re-starting and again stopping in order to produce the vibration itself.
Between these two phases, only the rotation speed for the shafts bearing the eccentric weights undergoes a change, while the conditions for the remainder of the process are unaltered.
With an operating time of one minute, the need arises for a dozen, if not more, start and stop phases, and the power supply that is installed is extremely important. The solution, which would consist of using variable vibration speeds, which are independent of the system of motors, would involve enormous speed regulators. Existing regulators are, moreover, not designed for such a function.
Given these inadequacies of the prior art apparatus, it was proposed that adjustable weights be positioned on the rotating shaft. However, due to the conditions of vibration, play quickly developed in the adjustable connection for the weights on the shaft and, due to the desirability of extended service life, this solution has been ruled out due to reduced service life.
A staggering of the weights or eccentric weights by means of cluster and spider gears lubricated by oil baths located within housings was also proposed in the prior art. This technique considerably limits the rotational speed of the shafts bearing the weights or eccentric weights. Also, the maximum vibration frequency achieved in this manner is not compatible with the production of very compact products such as curbs and paving materials, for which a frequency of 75 Hz (or 4500 revolutions/minute) is the required minimum.